Engendering a Reactive Uranium(Ⅲ) Center with a Single Pocket for Reactivity: A Combined Synthetic, Spectroscopic, and Computational Study

摘要

Although bonding in f-elements is traditionally described as mainly electrostatic, the issue of covalency remains an important subject of debate. In order to answer fundamental questions regarding trends in bonding and the reactivity of uranium and other actinide metal compounds, the discovery and detailed investigation of new molecules is necessary. In our efforts to identify and isolate uranium complexes with enhanced reactivity relevant to binding, activation, and functionalization of small molecules, we are currently investigating the coordination chemistry of uranium metal centers stabilized by classical Werner-type ligands. The stabilizing ability of "classic" macrocyclic amines has made this class of chelators an indispensable tool for transition-metal coordination chemistry. However, the coordination chemistry of uranium complexes with macrocyclic polyamine ligands remains largely unexplored. This is in contrast to the thoroughly investigated organometallic chemistry of uranium with cyclopentadienyl ligands and their derivatives as well as the recently developed amido chemistry of uranium. On the occasion of the Plutonium Futures 2003 conference, we wish to report on the detailed synthetic, spectroscopic, and computational investigations of analogous uranium Ⅲ, Ⅳ, Ⅴ, and Ⅵ derivatives supported by the tris-aruloxide functionalized triazacyclononane ligand, 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane ((ArO)_3tacn). We will demonstrate that the ancillary polyamine macrocycle provides a unique platform for enhanced reactivity at an electron-rich uranium center. Specifically, we will show that the introduction of ((ArO)_3tacn) to a uranium(Ⅲ) metal center results in the formation of stable core complexes leaving a single reactive coordination site available for ligand substitution reactions and redox events associated with small molecule or organic functional group activation.